Methane (CH4) and non-methane total hydrocarbons (NMHC) abound in nature, especially the atmosphere where CH4 and NMHC demonstrate concentrations of approximately 1.8 ppmv and 30 ppbv, respectively. The term non-methane total hydrocarbons (NMHC) refers to all hydrocarbons except methane and features extremely complicated compositions which mainly originate from combustion of gasoline, ignition of rubbish, evaporation of solvents, and the refining of waste. When existing in the atmosphere and exceeding a specific concentration, NMHC is not only harmful to the health of human beings but also gives rise to chemical smog upon exposure to sunlight under certain conditions to thereby bring about hazards to the environment and human beings.
Considering a wide variety of hazards caused by NMHC in the atmosphere, it is necessary to exercise real-time surveillance and control over pollutant sources, roads with heavy traffic, industrial parks and factories in terms of NMHC concentrations. The surveillance of NMHC content in the air began in the 20th century overseas by condensation-weight technique, combustion technique, and infrared spectrophotometry. Nonetheless, all these techniques have their own drawbacks, such as limits of use and intricate operation, and thus is never widely applied.
Nowadays, the detection and measurement of NMHC is carried out in various ways, including undergoing oxidation and reduction to CH4 before being detected and measured, undergoing oxidation and conversion to remove non-methane total hydrocarbons, and undergoing gas chromatography.
The aforesaid technique of converting NMHC into CH4 before being detected and measured involves carrying out semi-automated analysis of NMHC by following the steps described below. The gases CO, CO2 and CH4 are extracted from non-methane organic matters with a chromatography column. Then, the extracted non-methane total hydrocarbons are oxidized to become CO2, and the CH4 is restored by reduction, which are then measured with a CH4-oriented FID detector to thereby calculate the non-methane total hydrocarbon content. However, the aforesaid technique has its own drawbacks. First, the measurement process is subject to interference from CO2 and water. Second, the method requires catalytic oxidizing agents and reducing agents, and thus it is necessary to ensure that, during the online surveillance process, the oxidation rate of converting the non-methane total hydrocarbons into CO2 by oxidation and the reduction rate of converting CO2 into CH4 by reduction must be placed under intricate systemic control, thereby affecting the analysis process greatly.
The aforesaid technique of undergoing oxidation and conversion to remove non-methane total hydrocarbons is described as follows: a sample gas enters an analysis instrument through an admission pipeline, passes through a high temperature conversion furnace, wherein the high temperature conversion furnace is loaded with a conversion agent whereby all the non-methane total hydrocarbons of the sample gas are completely destroyed at 150-250° C., such that only the residual methane will undergo the test. Another gas route does not require the sample gas to pass through the high temperature conversion furnace, such that the gas being monitored contains all the substances, that is, total hydrocarbons. Given the timed switching of an electromagnetic valve, the two gas routes eventually undergo a hydrogen flame ionization detector test to thereby yield the methane concentration and the total hydrocarbon concentration, respectively. The difference in readings between the two gas routes equals the non-methane total hydrocarbon content. During the above process, a high temperature conversion furnace is required and adapted to destroy all the non-methane total hydrocarbons; however, the result of the test depends greatly on the degree of conversion and destruction of the non-methane total hydrocarbons during the above process.
Gas chromatography manifests much flexibility and is generally divided into two categories, namely total hydrocarbon-methane indirect test method and methane-non-methane total hydrocarbon direct test method.
The total hydrocarbon-methane indirect test method is in wide use and requires a sample gas to follow two routes, that is, one which involves measuring the total hydrocarbon content with the FID detector directly, and the other which involves passing the sample gas through a methane molecular sieve so as to remove high-carbon hydrocarbons, measuring the methane content with the FID detector, and eventually subtracting the methane content from the total hydrocarbon content to obtain the non-methane total hydrocarbon content. This method is recommended by China's Environmental Protection Administration (EPA) for use in measuring non-methane total hydrocarbon content and monitoring pollution sources which produce non-methane total hydrocarbons.
However, this method has its own drawbacks. For example, it requires subtraction when measuring the non-methane total hydrocarbon content, that is, subtracting the methane content from the total hydrocarbon content to therefore obtain the non-methane total hydrocarbon content. The basic methane concentration in the air sample is typically 1.8 ppmv approximately. If the basic non-methane hydrocarbon concentration in the air sample is much less than the basic methane concentration in the air sample, that is, when the methane concentration approaches the total hydrocarbons concentration (THC), the direct subtraction will bring about the addition of the errors of the readings, and in consequence the data error of the non-methane total hydrocarbons at a very low concentration increases.
On the other hand, the methane-non-methane direct test method is carried out by following the steps described below. The sample gas is adsorbed with an adsorption pipe. The methane passes through the adsorption pipe and then undergoes reverse desorption at the adsorption pipe. The organic matters which have undergone decomposition and desorption are indicative of the non-methane total hydrocarbon content. After the sample gas thus entered has undergone the quantification process performed in just one single instance with a quantification pipe, the test conducted on the methane and non-methane total hydrocarbons is done. This method is easy to carry out. However, the result of the monitoring of non-methane total hydrocarbons depends directly on such factors as to whether the desorption and reversing processes are carried out thoroughly to heavy non-methane organic matters at a constant temperature when the monitoring process is carried out continuously; as a result, the readings of the non-methane total hydrocarbon content tend to be low. In addition, the aforesaid thermal decomposition process necessitates high requirements for a thermal decomposition pipe and thus is susceptible to cross pollution.
With all things considered, it is necessary to provide a highly sensitive and highly stable online analysis method and apparatus which yield accurate surveillance results and are easy to operate in performing real-time online surveillance on airborne non-methane total hydrocarbons at pollutant sources, roads with heavy traffic, industrial parks and factories.